Biostimulants in agriculture

The past decades have witnessed tremendous growth in the use of biostimulants in agriculture and it is estimated that biostimulants will grow to $2 billion in sales by 2018 (Calvo et al., 2014). Recognizing the need to establish a legal framework for the marketing and regulation of these products the European biostimulants industry council (EBIC, 2012) defined plant biostimulants as " containing substance(s) and/or microorganisms whose function when applied to plants or the rhizosphere is to stimulate natural processes to enhance/benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and crop quality. " There is a clear need to improve our understanding of biostimulant function so that the efficacy of these materials can be improved and the industrial processes can be optimized. Determining the function of this class of products, however, has proven to be immensely difficult (Khan et al., 2009; Carvalhais et al., 2013; Rose et al., 2014). This is in large part due to the diversity of sources of these materials and the complexity of the resulting product, which in most cases will contain a significant number of poorly characterized molecules. Since biostimulants are derived from an incredibly diverse set of biological and inorganic materials (Calvo et al., 2014) including microbial fermentations of animal or plant feedstock, living microbial cultures, macro, and micro-alga, protein hydrolysate, humic, and fulvic substances, composts, manures, food, and industrial wastes prepared using widely divergent industrial manufacturing processes, it is illogical to assume that there is a single mode of action. The definition of biostimulants adopted by EBIC specifies that these materials should not function by virtue of the presence of essential mineral elements, known plant hormones or disease suppressive molecules. Accepting this definition, we hypothesize that biostimulants benefit plant productivity by interacting with plant signaling processes thereby reducing negative plant response to stress. This hypothesis recognizes the wealth of recent research demonstrating that plant response to stress is regulated by signaling molecules that may be generated by the plant or its associated microbial populations (Marasco et al., 2012; Bakker et al., 2014; Vandenkoornhuyse et al., 2015). Biostimulants may either directly interact with plant signaling cascades or act through stimulation of endophytic and non-endophytic bacteria, yeast, and fungi to produce molecules of benefit to the plant (Figure 1). The benefit of the biostimulant is derived from the reduction in assimilates that are diverted to non-productive stress response metabolism. In this research topic the effects …